1. Field of the Invention
The invention relates to a discharge lamp bulb that is used in a vehicle headlight or the like.
2. Description of Related Art
Japanese Patent Application Publication No. 2003-123630 (JP 2003-123630 A) discloses, as shown its FIG. 1 through FIG. 3, a discharge lamp bulb (discharge lamp device) that includes an arc tube body in which an arc tube where a pair of opposing electrodes are disposed inside a sealed glass bulb is formed inside a shroud glass in one body and an insulating plug which holds the arc tube body through a metal perpendicularly holding member. The arc tube emits light with a luminescent material that is filled inside the sealed glass bulb and electrodes, and the outgoing light from the center of the sealed glass bulb is reflected forward by a reflector that is arranged to the rear of the sealed glass bulb obliquely above the sealed glass bulb. An amount of light reflected by the reflector increases as an angle that is formed by a straight line passing through a luminescent center of a luminous tube and perpendicular to a central axis of the luminous tube and a straight line passing through the luminescent center and an innermost edge of the reflector (maximum angle of incidence of the light that goes from the luminescent center toward the reflector) increases.
In recent years, since further size and weight reductions for vehicle headlights are demanded, further size reduction for the discharge lamp bulb is also demanded. The size reduction for the discharge lamp bulb can be achieved by shortening the length of the arc tube body that is shown in FIG. 2 of JP 2003-123630 A.
However, when projecting length of the arc tube body from the insulating plug is shortened, the luminescent center of the arc tube body shown in the drawing is arranged closer to the insulating plug than that of the arc tube body that is shown in FIG. 2 of JP 2003-123630 A. As a result, in the discharge lamp device in the drawing, the incident angle from the luminescent center to the reflector decreases, and therefore the amount of light reflected by the reflector decreases as compared to that of the discharge lamp bulb that is disclosed in JP 2003-123630 A.
Furthermore, when the luminescent center of the arc tube body is arranged closer to the insulating plug, because part of the outgoing light from the arc tube body to the reflector is blocked by a tab piece of the metal perpendicularly holding member or the insulating plug in the discharge lamp bulb, the amount of light reflected by the reflector is reduced.